Optical coherence tomography (OCT) is a high-resolution medical and biological imaging technology. OCT has been used in ophthalmology for high-resolution tomographic imaging of the retina and anterior eye. Recently, the technique has been applied for imaging a wide range of nontransparent tissues to investigate applications in tissues studies and medical applications in gastroenterology, urology, and neurosurgery. OCT detects the reflections of low-coherence light, and cross-sectional imaging may be performed by measuring the backscattered intensity of light from structures in tissue. This imaging technique is attractive for medical imaging because it permits the imaging of tissue microstructure in situ. In situ imaging with OCT may provide micron-scale imaging resolution without the need for excision and histological processing.
Spectroscopic optical coherence tomography (SOCT) is an extension of OCT that can provide depth resolution and can differentiate between different types of tissue. In addition to the normal OCT measurement of the intensity of light backscattered from the sample, SOCT measures the spectral absorption and reflectance data from the tissue. Tissue structure can be resolved based on local optical densities, ignoring the frequency dependent changes. SOCT resolves both the amplitude, which contains the density information, and the frequency, which contains the spectroscopic molecular composition information.
Contrast agents may be used to improve the resolution of images obtained from an imaging technique, including OCT. Conventional contrast agents serve to increase the intensity of backscattered light. For example, air-filled micro-bubbles and engineering microspheres may be introduced into tissue to increase the back-scattering from tissue. In another example, a molecular contrast agent can be generated using a pump-probe technique to change the absorption.
A method to increase the types of tissue that may be resolved with SOCT methods would be beneficial. For example, substances such as melanin and hemoglobin exhibit strong selective absorption signature, and may be directly resolved by conventional SOCT. However, these substances are common in tissue and often may not be used to discriminate tissue types. It would be desirable to provide contrast agents that could improve and expand the application of SOCT. It would also be desirable to extract additional information from tissue samples regarding the structure and the composition of the tissue.